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Stable free-radical complexing reagents in applications of electron spin resonance to the determination of metals
Analytica Chimica Acta, 128 (1981) 85-99 Bisevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
STABLE FREE-RADICAL COMPLEXING REAGENTS IN APPLICATIONS OF ELECTRON SPIN RESONANCE TO THE DETERMINATION OF METALS Part 2. Spin-Labelled Iminooxime [ 11
V. Yu. NAGY, M. V. EVSTIFEROV, Yu. A. ZOLOTOV*
0. M. PETRUKHIN,
Vernadskii Institute of Geochemistry Sciences, Moscow, I I7334 (U.S.S.R.)
and Analytical
L. B. VOLODARSKII
Chemistry,
U.S.S.R.
and
Academy
of
(Received 28th January 1981)
SUMMARY Possibilities for the determination of metals by means of the intensity of the e.s.r. signal of the chelate-forming reagent, spin-labelled iminomonoxime - 4_oximinomethyl2,2,5,5-tetramethyl-3-imidaxoiine-l-oxyl, have been studied. The dissociation constant of the oxime group (pKz = 9.36 f 0.08) and the reagent partition constant in the chloroform-water system (log If0 = 0.80 f 0.11) are reported. The reagent extracts copper, cobalt and nickel into chloroform. Copper is extracted as its CuA, chelate (log KD = + 0.09; log & = 15.8 + 0.2). Several properties of the spin0.91 2 0.03; log rc, = -3.35 labelled and conventional oximes are compared. It is confirmed that a rzdicalcontaining substituent produces a strong electron-acceptor effect. Unusual extractive and e.s.r.spectroscopic behaviour of cobalt is indicated; an adduct of the spin-labelled chelate with atmospheric oxygen seems to be formed. Methods for the determination of cobalt and nickel based on the extraction with spin-labelled oxime into chloroform and suhsequent separation of the excem of reagent on a chromatographic column are described_ The detection Limits are 3 x lo-’ M for cobalt and lo* M for nickel.
The progress of synthetic organic chemistry has made it possible to obtain
reagents whose molecules contain a chelate-forming group capable of bonding metals and a stable unpaired electron. Of special importance are the reagents based on nitroxyl radicals that retain the unpaired electron for a practically
indefinite period of time. These compounds make it possible to extend substantially the possibilities of electron spin resonance as a method of determining metals, raising its sensitivity and broadening the range of the elements determined [ 23. In earlier work [ 1, 23, the characteristic features of such applications were examined in detail, and a review of literature on related problems was presented. In a continuation of the investigation of analytical applications of these compounds [l-6], the analytical properties of the nitroxylcontaining iminomonoxime compound, 4-oximinomethyl-2,2,5,5-tetramethyl-3imidazoline-1-oxyl (I, designated below as temioxime) [ 71 have been studied. 0003-2670/81/0000-0000/$02.50
0 1981 Elsevier Scientific Publishing Company
66
Temioxime is capable of bonding metal ions by coordination to the iminooxy nitrogen and the heterocyclic nitrogen with the formation of a fivemembered ring_ Some of these chelates were synthesized in aqueous medium and investigated by physical methods [ 81. An extraction-radiospectrometric method for the determination of palladium with this reagent was reported [4] . The aim of the work described here was to investigate more comprehensively the analytically important properties of temioxime and to assess its possibilities as a reagent for other metals. This systematic study also provided an opportunity to check the general conclusions about the effect exerted by the spin label on the properties of an extractive reagent that were drawn in the study of spin-labelled p-diketones [l] . EXPERUVIENTAL
Reagent
Temioxime synthesized by one of us (L.B.V.) at the Novosibirsk Institute of Organic Chemistry was purified chromatographieally on a silica gel column (LS 5f40 pm; Lachema, Bmo) using a mixture of ethyl acetate with chloroform (2 + 1) as the mobile phase. The purity of the compound was checked by occasional chromatography on plates with the same grade of silica gel using various organic solvents_ Storage of the reagent for one year did not cause any significant decomposition_ Temioxime solutions were prepared daily by dissolving an accurately weighed portion of the purified reagent in chloroform. The solutions and the solid reagent were stored in dark vessels in a refrigerator.
Extraction
of metals
The extractions of copper, cobalt, nickel, zinc and manganese were investigated by shaking aqueous solutions of the metal with chloroform solutions of the reagent in ground-glass stoppered tubes. Distribution ratios for copper, cobalt, zinc and manganese were determined radiometrically (60Co, 64Cu, 54Mn, 652n)_ The extraction efficiency for nickel was characterized by the decrease of the radioactivity (63Ni) in the aqueous phase in the course of extraction, using a ZhS-7 liquid scintillator.
Chromatography The components of extracts were separated by liquid adsorption chromatography on columns of various sizes containing silica gel LS 5/40 pm. Separation conditions were established by chromatography on a thin layer (0.5 mm) of the same sorbent on 9 X 12-cm glass plates. Chromatograms
were detected with iodine vapour as well as with an ethanolic solution of dimethylglyoxime (detection of nickel-containing spots). Various organic solvents were used as mobile phases. AI1 clzzomatographic experiments were conducted at room temperature.
Instrumentation
E-W_ spectra were recorded with an RE-1306 radiospectrometer at room temperature using ampoules 1 mm in diameter. The g values were determined relative to diphenylpic~~ydr~yl~ magnetic field sweep was checked by manganese(I1) spectra in magnesium oxide powder. The same sample was used as an adjacent concentration standard. Absorption spectra of reagent solutions in the visible and ultraviolet regions were recorded with an SF-16 spectrophotometer with l-cm cuvettes. pH values were measured with a glass electrode using a pH-673 potentiometer. In all experiments the reagents used were of “Pure for Analysis” or “Chemically Pure” grade. Chloroform was purified by six-fold scrubbing with an equal volume of distilled water followed by drying over calcium chloride and distillation.
Procedure
for determination
of cobalt
Place 2 ml of cobalt solution (4 X lo- ‘-1 X IO-* M) in borate medium (pH[ 8.5) into a tube with a ground-glass stopper, add 2 ml of a 10W3M solution of temioxime in chloroform and shake the tube for 5 min. After phase separation, transfer the organic phase to a column with dry LS 5140 I.trnsilica gel compacted as much as possible (bed height 75 mm; diameter 6 mm), Pass a (1 -!- 2) mixture of chloroform and ethyl acetate through the column at a rate of 0.5 ml min-’ under pressure of nitrogen from a gasbag. It is convenient to use long graduated glass columns so that 11 ml of eluent can be added in a single portion and the progress of the elution can be checked easily_ Control the eluent consumption rate by the volume markings on the tube. (Repeated tests showed that this system provided good reproducibility.) Discard the first 7.5 ml of eluate. Collect the subsequent 3 ml; after stirriig, place this solution in an ampoule (1 mm diameter) and record the e.s.r, spectrum at a moduIation amplitude of 1.0 X 1.0 and a h-f. osculations attenua~on of 7 dB, Take the ratio between the amplitude of the complex spectrum derivative and the amplitude of the adjacent standard spectrum derivative (see below) as the measure of signal intensity. For small cobalt contents, evaporate the cobalt-containing fraction of the eluate in a stream of warm air and dissolve the dry residue in 3 ml of chloroform. Determine the cobalt concentration from a previously plotted calibration graph.
Procedure
for determination
of nickel
Place aqueous solutions into a tube with a ground-glass stopper so that
88
the concentrations in the final volume of 2 ml are 0.1 M sodium perchlorate, and 0.01 M sodium acetate, 0.001 M sodium hydroxide and (I-20) X IOm6M nickel. Add 2 ml of 0.01 M solution of temioxime in chloroform and shake
the tube for 40 min. Separate the organic phase, place 2 ml of the same temioxime soiution in the tube and again shake for 40 min. PIace the combined organic phases in a sintered grass filter funnel covered with a 2-mm layer of LS 5/40 pm silica gel protected with a paper filter. Wash the silica gel with 30 mf of ethyl acetate-chloroform mixture (2 + I) and then pass 30 ml of ethanol through it. Evaporate the ethanol solution in a stream of warm air and dissolve the dry residue in 2 ml of chloroform.
Fill an ampoule
(l-mm diameter) with the solution and place it in the resonator of the spectrometer. Record the es-r_ spectrum at a modulation amplitude of 0.7 X 1.0 and a h.f, oscillations attenuation of 7 dB_ Take the ratio between the mean amplitude of the first derivative (for three spectral lines) and the mean amplitude of the first derivative of the adjacent standard lines (see below) as the signal value. Determine the nickel concentration from a previously plotted calibration graph. RESULTS AND DISCUSSION
Recgent behaviour Acid-base properties_ These properties were studied spectrophotometrically_ fn alkaline media, the U.V. spectrum of the reagent has an intensive band with a maximum at 267 nm (Fig. 1); its location is identical with that of the bands of dissociated forms of other oximes f93. From the dependence of the absorbance of the solution at 267 nm on pH, the dissociation constant of the oxime group [lo] (21”C, 1 X 10e4 M temioxime, I = 1 cm, n = 7, CLI = 0.05) was found to be nK% = 9.25 + 0.08 at ionic strength 0.1 (NaClO,) or PC = 9.36 i 0_08. In addition to this acid-base equilibrium for temioxime, protonation of the nitrogren atom in position 3 of the heterocycfe as well as protonation of the nitroxyl group can be assumed. However, it proved impossible to investigate these processes: they seemed to take place at pH values lower than 2, where absorbances were unstable with time. When the spin-labelled @&ketone was investigated [If , it was concluded that the t~~methyl~mid~ol~eoxyl substituent was very electronegative; according to the data obtained, the electronegativity is commensurable with that of the trifluoromethyl group. Comparison of temioxime with other monoximes with respect to the dissociation constant confirms this conclusion. Indeed temioxime has much stronger acidic properties than, e.g., salicylaldoxime (pK, = 12-f), and its acidity is close to that of monoximes containing highly electronegative acyl groups (diacetylmonoxime 9.53; or-furylmonoxime S-67; a-benzy-hnonoxime 8.60). Reagent distribution in the chloroform-water system. Figure 2 shows the dependence of the temioxime distribution ratio (D) on pH in the chloroform-
90
the conclusion on the closeness of molar volume glyoxal aldoxime confiis and phenyl made when flincre,33ents for tetramethylimidazolineoxyl diketones were examined. of metals The extracting capacity of temioxime was tested for five metals: copper, cobalt, nickel, zinc and manganese. The last two were not extracted into chloroform under any conditions, including the presence of strong donoractive compounds and large poorly hydrated ions. Copper. Copper was extracted to a considerable degree into chloroform over a rather broad pH range (Fig. 3)_ The best extraction was achieved at pH 7-10, but even under these conditions and with a 500-fold excess of reagent, not more than 90% of the metal was extracted_ The dependence of copper distribution ratio on temioxime concentration (Fig. 4) shows that it is impossible to improve radically the extraction by a simple increase in reagent concentration. The distribution ratio was not increased by the addition of tributylphosphate. The slopes of the ascending parts of the plots in Figs. 3 and 4, which are close to 2, indicate that the extraction reaction at pH below 9 can be described by
Extraction
Cu2+ + 2HA,,,
= CuAz(,, + 2H’
where H-4 is the temioxime molecule. ~~athematic~ treatment of the experimental data, taking into account competitive complexing with acetate ions in the aqueous phase [11] (pKI = 1.67; pKz = 0.98; pK3 = 0.42; plz, = -0.19 [ 12]), made it possible to obtain the following values of the constants: log K,, = -3.35 c 0.09 (n = 5, 01= 0.05); log KD(CuAz) = 0.91 f 0.03 (n = 14; a = 0.05). Substitution of these
I
4
6
8 OH
10
12
10-4
to-3
10-2 lo-’
Temtoxime concentration (Ml
Fig_ 3, Dependence of copper distribution ratio on pH: IOms M topper( temioxtie; 0.01 M sodium acetate; JX= 0.1 (NaCIO,). Fig. 4. Dependence of copper distribution ratio on temioxime copper; pH 6.8 (0.01 M sodium phosphate); w = 0.1 (NaClO,).
5 X
concentration;
1W3 M lo-’
M
91
values in theequation& = K,K2,(HA)/R~Rn(CuA2) [ll] resulted in a value for the logarithm of the total stability constant for the extracted complex in the aqueous medium: log pz = 15.8 f 0.2. This value can be compared with the stability constants of conventional copper(I1) oximates in Table 1. Copper(I1) temioximate is inferior in stability to dioximates but is close to carbonyloxirnates. Temioxime is also similar to carbonyloximes in the dissociation constant of the oxime group. This confirms the conclusion (made quantitatively in studying spin-labelled P&ketones) that the electron-acceptor effect of the radicalcontaining substituent not only decreases the pK, of the reagent but also causes a corresponding decrease in the stability of the complexes with metals, which, in turn, changes the extractive efficiency. A serious obstacle for the analytical application of this extraction system lies in frequent and unreproducible disturbances of material balance relative to copper that cannot be avoided by introducing weak complexing reagents_ Cobalt. The dependence of the cobalt distribution ratio on pH (Fig. 5) is characterized by a sharp increase in extraction at pH 7.5: over a range of less than 0.2 pH units the distribution ratio increases by more than two orders of magnitude_ The clearly defined plateau observed immediately after the rise is not wide, but the distribution ratios are large: they make it possible to extract in one step more than 97% of cobalt (with a lOO-fold excess of reagent and equal phase volumes). At pH values higher than 9.5, the extraction becomes impossible because of hydrolysis. The dependence of the cobalt distribution ratio on temioxime concentration (Fig. 6) shows that the extraction efficiency cannot be improved by raising the temioxime concentration above a certain proportion; distribution ratios on the plateaux of both plots indicate that the partition constant of the extracted complex is log Ro = 1.75 f 0.05 (n = 10; a! = 0.05). The slope of the ascending part of the plot is 2, showing a 2:l molar ratio between the reagent and the metal in the extracted complex. The cobalt distribution ratios under the optimal conditions are not improved by addition of tributylphosphate (TBP): at small and medium concentrations, TBP had no effect, whereas at 0.1 M concentrations the extraction became worse. The cobalt extraction at pH 8.5 with a loo-fold excess of reagent is not TABLE 1 Stability
constants
of copper
oximates
in aqueous
media
(2S’C)
[ 91
Reagent
COXlditi0Il.S
lotz Pz
Reagent
Conditions
lolz b.
Phenylglyoral aldoxime Diacetylmonoxime a-Benzyhxmnoxime a-Furylmonorine 1-Nitroso-2-naphthol 2-Nitroso-1-naphthol
0.1 M NaClO, .. .. .. .. ..
10.70= 15.91= 13.70= 9.46= 14.40= 15.60=
Nitroso-R-salt Nitroso-HaAt Dimethylglyorime Methylethylglyoxime 1.2Cyclohexanedionedioxime
0.1 M KC1 W-+0 !J +O.l 0.1 M N&IO, 0.2 M N.&IO,
15.oc 13.oc 18.5C 19.6; 20.3s
aDetermined by zn extractionmethod.bDeterminedpotentiometrically.
I
I 4
5
6
7
8
9
IO
PH
Fig. 5. Dependence of cobalt distribution ratio on pH; lo-’ P = 0.1 (NaClO,).
IO
-4
Temoxme
-3 IO
-2 IO
IO
concentmhon
(Ml
-I
M cobalt; lo-” M temioxime;
Fig. 6_ Dependence of cobalt distribution ratio on temioxime concentration; lo+ M cobalt; P = 0.1 (NaCIG,); 5 x lo-’ M sodium tetraborate.
pH 8.4;
affected by very large amounts of borate or even tartrate ions. This permits reliable standardization of extraction pH by using, e.g., a borate buffer. The following conditions can thus be regarded as optimal for cobalt extraction with temioxime into chloroform: pH 8.5 (10m3M borate solution), 100-fold excess of reagent at ionic strength 0.1 (sodium perchlorate). Under these conditions, equilibrium is established in less than 5 min of shaking. Nickel. Radiometric investigation of nickel extraction was hampered by the extremely low radiation energy of the 63Ni radioisotope; liquid scintillation counting was therefore used. The extraction of nickel into chloroform starts only at relatively high pH values (Fig. 7). Since in these conditions (especially above pH 9) there is noticeable hydrolysis, the extraction was conducted in the presence of acetate ions. This made it possible, without any appreciable reduction of recovery, to avoid nickel losses even at pH 11. Extraction efficiency increases with increasing pH, but (if a loo-fold excess of reagent is used) even at pH 10.8, recoveries do not exceed 50%. A 500fold excess of reagent improves the efficiency (Fig. S), but larger excesses give no benefit. Nickel recovery is somewhat increased when more acetate is added. Donor-active compounds (pyridine, TBP) do not affect the extraction efficiency; a slight increase was observed when 1 M levels of additives were used, but phase separation was then poor. The extraction was not rapid: with 2 X 10e6 M nickel and 10d3 M temioxime under the conditions used for Fig. 8, the system reached true thermodynamic equilibrium on shaking for 30 min, the recovery being 75%. The optimal conditions of nickel extraction are: pH 10.8, at least a 500-
93
80. 0 60
40.
20 .I 4
6
8 PH
IO
12
lo* Temioxire
[CT4
1o-3
lo-*
concentmtion
(Mb
Fig. 7. Dependence of nickel extraction on pH; IO-’ M nickel; 0.01 M temioxime; 0.01 M sodium acetate; c1= 0.1 (NaClO,)_ Fig. 8. Dependence of nickel extraction on temioxime concentration; pH 10.8; 2 nickel; 10e4 M sodium acetate; JJ= 0.1 (NaClO,).
X
lo-‘M
fold excess of temioxime, a 500-fold (relative to nickel) excess of acetate ions and 0.1 M sodium perchlorate. About 75% of the nickel is then recovered in one extraction_ After a second extraction, about 95% of the metal is recovered, which is acceptable for a relative procedure to be worked out_ Development mina tions
of OR extraction-radiospectrometric
method
for metal deter-
The necessary prerequisites for the development of methods of metal determinations with the help of a spin-labelled reagent are the completeness and the reproducibility of extraction. Of the three extraction systems investigated, those for cobalt and nickel were suitable for further study. Another important condition is the possibility of separating the signal of the spinlabelled complex of the determined metal from the reagent signal present in the excess [Z] _ The enormous excess of reagent in the extract excludes the possibility of such separations by purely radiospectrometric means. Comparison of the pH dependences of the distribution ratios for the cobalt chelate and temioxime itself shows that there are no extraction conditions suitable for their separation_ Moreover, the excess of reagent could not be removed efficiently by scrubbing the extracts with alkaline solution. The chromatographic separation of the components of the cobalt- and nickel-containing extracts was therefore examined. Cobalt. Chromatography of a cobalt-containing extract in a thin silica-gel layer showed two new components in addition to the excess of temioxime. Thus, when ethyl acetate was used as the mobile phase, the temioxime spot was characterized by Rf = 0.74, and the two new components by Rf = 0.50 and Rf = 0.02. Radiometric investigation proved that both these components contained cobalt, the ratio between their amounts depending on the extraction
94
conditions_ When an aqueous 10m3M cobalt solution (pH 8.5, borate solution) was shaken with 10d3 M temioxime solution in chloroform, the radioactivity ratio between the less and more mobile components was 3:7. However, when cobalt was extracted from lO-6-lO-5 M solutions with a 10q3 M reagent solution in chlcroform, the fraction of component remaining at the’start was approximately constant, amounting to about 3% of the total amount of metal. These cobalt-containing components were isolated from the extract on a sihea gel column by consecutive use of ethyl acetate (separate elution of temioxime and the more mobile coba.lt-containing component) and then acetone (elution of the second component). E.s.r. spectra of these complexes are shown in Fig. 9. The e.s.r. spectrum of the less mobile component is the quintet usual for a nitroxyl~on~~g complex of a diamagnetic ion with exchange interaction of nitroxyls, and coincides with that described for the CoA3 complex (A is temioxime anion) obtained in the aqueous phase (81. The spectrum of the second component (the main one in the conditions of quantitative extraction) has no analogues among the described spectra of nitroxy1containing metal complexes_ This spectrum could have been assigned to the spin-labelled CoAz complex of paramagnetic cobalt(I1) with exchange interaction between the electrons of nitroxyls and the metal ion_ It is, however, difficult then to explain why the compIex should be obtained in greater amounts at lower cobalt concen-
Fig. 9. E.s.r. spectra of extract components after the extraction of cobalt with temioxime: (-----) temioxime; ( -) the main cobalt-containing compound; (----*) admixed(low-
mobility)cobaltcomplex_
95
trations in the aqueous solution; the abrupt rise in the distribution ratio at pH 7.5, and the lack of dependence of the extraction efficiency on donor additives, cannot be explained_ These contradictions can be removed if it is assumed that the extracted compound is an adduct of the chelate with atmospheric oxygen, CoA,O;, where cobalt(fII) is present with oxygen as the 0; anion because of electron density delocalization 1133. This assumption is in agreement with the unusually abrupt change in the distribution ratio with increasing pH, the slope of the log DC0 = f(CnA) dependence, the dependence of the percentage of this compound on .the overall cobalt concentration, and the unusual e.s.r. spectrum. Moreover, this corresponds to the behaviour of coba& in other similar systems, particularly dioxime systems C% 141. It is thus clear that under the conditions of quantitative extraction, the complex which has an e.s.r. spectrum with a quintet is formed only in a relatively small constant proportional amount. This compound is readily separated from the main compound on a chromatographic column. All this makes it possible to develop a method for cobalt determinations based on spectrometry of tkresecond compound only. Attempts TV determine cobalt from the signal of this compound against the background of re agent excess were unsuccessful, although the spectrum contained portions where free tsmioxime absorbed insignificantly. The problem was that complete. separa%on of the signal from the reagent in 100lOOO-fold excess required the use of extremely small amplitudes of magnetic field strength modulation; such amplitudes result in a sharp decline of the sensitivity of the e.s_r. method as a whole. Tests of various mobile phases for the chromatography of the extracts showed that the best separation of the main complex, the reagent excess and the admixed complex was accomplished with a chloroform-ethyl acetate mixture (1 + 2). The Rf values were 0.63 (reagent), 0.21 (main complex) and 0.02 (admixed complex) and the spots were very compact. The extract was separated for e.s.r. spectrometry on a LS 5/40 pm silica gel column (75 mm X 6 mm). The column was dry-filled, with the silica gel being compacted. To speed up the separation, a nitrogen pressure of about 700 hPa was applied at the top of the column. The optimal elution rate was 0.5 ml min-‘. The elution curve obtained (Fig. 10) shows complete removal of the reagent in 7.5 ml of the mobile phase, and elution of the cobalt complex in the next 3 ml. The cobalt-containing fraction of the eluate can be used directly to record the e.s.r. spectrum, but the detection limit can be decreased if the ethyl acetate--chloroform mixture is replaced by pure chloroform after evaporation of the eluate to dryness. The effects of different conditions of e.s.r. spectra recording on the detection limit and the precision of cobalt determinations were investigated. The best results were obtained when the l-mm ampoule was completely filled with solution and submerged in the resonator as far as possible, and when its position was strictly fixed relative to both depth and vertical
96
*i
Fig. 10. Elution curve for the separation of cobalkontaining
extract.
orientation_ The amplitude of the signal from the complex increases monotonously with increasing amplitude of the magnetic field strength highfrequency modulation and decreasing attenuation of the high-frequency oscillations. However, with a fairly open attenuator the noise of the automatic frequency adjustment also increases, and the signal/noise ratio passes through a maximum at an attenuation of 7 dB. The optimal parameters for recording the spectra of the cobalt-containing complex on the RE-1306 radiospectrometer included a 7 dB attenuation of h.f. oscillaticns, and a modulation amplitude setting of 1.0 X 1.0; the time constant and magnetic field sweep rate were chosen depending on the range of cobalt concentrations, but the time of magnetic field sweep from the maximum to the minimum of the first derivative curve was always S-10 times higher than the time constant_ Under these conditions, the complex e.s.r. spectrum of the compound blended into a broad singlet. In the 3 X 1O-7-l X lo-’ M range, the line width did not depend on concentration, and the amplitude of the first derivative was directly proportional to the initial cobalt concentration in the aqueous phase, so that this amplitude served well as a measure of signal intensity. To exclude errors associated with uncontrollable variations in instrumental operation, the signal from the complex was correlated with that of the adjacent standard, manganese(U) in magnesium oxide. The limit of detection for cobalt on the RE-1306 e.s_.r. spectrometer without solvent substitution was 2 x 10e6 M (with respect to concentration in the initial solution prior to extraction). If chloroform is substituted in the final step, the detection limit can be reduced to (3-4) X 10S7 M. The
97
calibration plot for cobalt determination is described by the straight-line equation Z/&tancl = (0.52 + 0.08) X lo6 X Cc, + (0.24 f 0.54)
(n = 5, OL = 0.05)
The accuracy and precision of the method are characterized by the data presented in Table 2; these results were obtained by the faster procedure without solvent substitution. The mean relative error for the determination of 10m5M cobalt from one recording of the spectrum was 6.2%, whereas the error from three parallel recordings was 3.7%. Cobalt determinations were not affected by the presence of tenfold amounts of nickel and insignificantly affected by copper in the same amounts, but iron(II1) and lead interfered seriously. Determinations take 30 min each without solvent substitution and 40 min with solvent substitution. Nickel. Chromatographic investigation of a nickel-containing extract in a thin silica-gel layer showed that the nickel complex is sorbed very securely. The chloroform-ethyl acetate (1 + 2) eluent did not elute the chelate at all, but the excess of reagent moved as a very compact spot (R, = O-63), as shown above. Even such a polar solvent as ethanol eluted the complex only slightly (RI = 0.2). @n this basis, it was decided to use a sintered glass filter funnel to TABLE 2 Results of determining lo-* M cobalt with temiosime by the extraction-xx. No. of parallel tests
Co found from the result of single recording (x 1O-5 M)
1
0.95 0.99 0.86 1.09 1.03 0.96 1.06 0.91 l-10 1.16 0.97 0.99 0.99 1.03 1.08 0.98 0.90 O-94
2 3
4 5
6
method
Co found from the mean amplitude of three recordings (x 1O-5 M)
Relative error of determination from the mean amplitude (W)
1 14 9 3 4 6 9 10 16 3 1
0.94
6
1.01
1
1.05
5
1.04
4
;
1.01
1
0.95
5
Relative error of determination from single recording (a) 5
6 2 10 6
98
which a 2-mm layer of silica gel was applied. After the content of the extract had been sorbed by this filter, the excess of reagent was removed by passing through it 30 ml of the chloroform-ethyl acetate mixture (1 f 2). The complex remaining on silica gel was then washed out with 30 ml of ethanol, the ethanol was evaporated in a stream of warm air, and the dry residue was dissolved in chloroform for e.s.r. spectrometry. The spectrum of the nickelcontaining complex was a triplet of equdUy intense lines whose parameters practically coincided with those of the reagent spectrum (g = 2.0057 F 0.0002; aN = 1150 A m-l; AH,, = 220 m-l). The optimal conditions for the recording of this signal were similar to those for the cobalt determinations, except for a modulation ampiitude setting of O.‘i X 1.0. The calibration plot for nickel determinations by this method is described by the straight-line equation -A /Astind = (0.29 + 0.12) X IO6 X cm + (0.82 -t 1.18) (n = 5, (lr= 0.05) The limit of detection for nickel was 10S6 M. The mean relative error for the determination of 1.5 X 10-’ M nickel was 15% The error of e.s.r. spectrometry itself for concentrations of 2 X 10m6M amounts to 4%, thus the main sources of error are the operations preceding the final measurements. The most probable source of error seems to be incomplete dissolution of the residue in chloroform after the removal of ethanol. Nickel can thus be determined with the help of temioxime, but this method is inferior to the other extraction~.s.r. methods in a number of characteristics (rate of determination, detection limit and precision). CONCLUSIONS This study of the analytically important properties of the new spin-labelled reagent temioxime, has shown that temioxime differs essentially from the previow;y studied spin-labelled reagents in its high stability, which is comparable with that of conventional organic reagents. Suspicion that low stability is a property inherent in all radical-containing reagents is thus disproved. The investigation has confirmed that spin-labelled reagents can effectively extract metals into organic solvents. The evidence obtained confirms the statement that a spin-labelled substituent based on imidazoline has a strong electron-acceptor effect. This results in an increase in the dissociation constant of the chelate-forming group, with a corresponding change in the stability of the complexes with metals, and so influences the completeness and the regularities of metal extraction_ In other respects radical-containing reagents seem to behave as their more usuaf analogues. In the case of temioxime, it again proved impossible to obtain situations where one could, without a significant loss in sensitivity, determine the signal intensity from the complex against the reagent background signal. In using e.s.r. spectrometers of 3.2-cm range, separation of the excess of reagent seems to be a prerequisite for the development of highly sensitive analytical
99
methods based on nitroxylcontaining reagents. Investigation of temioxime has demonstrated the possibilities of chromatography as a rapid convenient method of extract separation. Finally, with cobalt and nickel as examples, it has been shown practically that application of the e.s_r. method via the signals of spin-labelled organic radicals provides an advantage in sensitivity even for metals that can be determined by their own pammagnetism [X,16]. REFERENCES
Yu. A. Zolotov, 0. M. Petrukhin, V. Yu. Nagy and L. B. Volodarskii, Anal. Chim. Acta, 115 (1980) 1. 2 Yu. A. Zolotov, in D. M. Carroll (Ed.), EuroanalysisIII, Reviews on Analytical Chem1
istry, Applied Science PubIishers,London, 1979, p. 13. 3 P. M. Solozhenkin, N. G. IUassenand F_ A. ShvengIer,Zh. Anal. Khim.. 32 (1977) 2080. 4 S. V. Larionov, V. I. Ovcharenko, R. A. Sadykov and L. B. Volodarskii, IZV. AN SSSR,
Ser. Khim_, (1975) 1922. 5 Yu. A. Zolotov, 0. M. Petrukhin, N. A. Kurdyukova, V. V. Zhukov and I. N. Marov, Zh. Anal. Khim., 33 (1978) 1307. 6 V. Yu. Nagy, 0. M_ Petrukhin, Yu. A. Zolotov and L. B. Volodarskii, Izv. Akad. Nauk SSSR, Ser. Khim., (1978) 2186. 7 L. B. Volodarskii, I. A. Grigor’ev and G. A. Kutikova, Zh. Org. Khim., 9 (1973) 1974. S V. I. Ovcharenko, S. V. Larionov, R. A. Sadykov, R. 2. Sagdeev, I. A. Grigor’ev and L. B. Volodarskii, Koord. Khim., 3 (1977) 1558. 9 V. M. Peshkova, V. M. Savostina and E. K. Ivanova, Oksimy (Oximes), Nauka, Moscow, 1977, (in Russian) pp. 35-37,70,76,96. 10 A. Albert and E. P. Serjeant, Ionization Constants of Acids and Bases, J. Wiley, New York, 1962, p. 69. 11 Yu. A_ Zolotov, Extraction of Chelate Compounds, Ann Arbor, Humphrey Sci. Pubis., Ann Arbor, 1970, pp_ 24,32,39. 12 L. G. SillLn and A. E. Martell, Stability Constants of Metal-Ion Complexes. The Chemical Society, London, 1964, p. 365. 13 B. M. Hoffman, D. L. Diemente and F. Basolo, J. Am. Chem. Sot., 92 (1970) 61. 14 D. Diemente, B. M. Hoffman and F. Basolo, Chem. Comnmn., (1970) 467. 15 T. Meisel and G. G. Guilbault, Anal. Chim. Acta, 50 (1970) 143. 16 P. M. Solozhenkin, G. G. Sidorenko and Z. R. Baratova, Dokl. Akad. Nauk Tadzh. SSR, 17 (12) (1974) 25.
Printed in The Netherlands
STABLE FREE-RADICAL COMPLEXING REAGENTS IN APPLICATIONS OF ELECTRON SPIN RESONANCE TO THE DETERMINATION OF METALS Part 2. Spin-Labelled Iminooxime [ 11
V. Yu. NAGY, M. V. EVSTIFEROV, Yu. A. ZOLOTOV*
0. M. PETRUKHIN,
Vernadskii Institute of Geochemistry Sciences, Moscow, I I7334 (U.S.S.R.)
and Analytical
L. B. VOLODARSKII
Chemistry,
U.S.S.R.
and
Academy
of
(Received 28th January 1981)
SUMMARY Possibilities for the determination of metals by means of the intensity of the e.s.r. signal of the chelate-forming reagent, spin-labelled iminomonoxime - 4_oximinomethyl2,2,5,5-tetramethyl-3-imidaxoiine-l-oxyl, have been studied. The dissociation constant of the oxime group (pKz = 9.36 f 0.08) and the reagent partition constant in the chloroform-water system (log If0 = 0.80 f 0.11) are reported. The reagent extracts copper, cobalt and nickel into chloroform. Copper is extracted as its CuA, chelate (log KD = + 0.09; log & = 15.8 + 0.2). Several properties of the spin0.91 2 0.03; log rc, = -3.35 labelled and conventional oximes are compared. It is confirmed that a rzdicalcontaining substituent produces a strong electron-acceptor effect. Unusual extractive and e.s.r.spectroscopic behaviour of cobalt is indicated; an adduct of the spin-labelled chelate with atmospheric oxygen seems to be formed. Methods for the determination of cobalt and nickel based on the extraction with spin-labelled oxime into chloroform and suhsequent separation of the excem of reagent on a chromatographic column are described_ The detection Limits are 3 x lo-’ M for cobalt and lo* M for nickel.
The progress of synthetic organic chemistry has made it possible to obtain
reagents whose molecules contain a chelate-forming group capable of bonding metals and a stable unpaired electron. Of special importance are the reagents based on nitroxyl radicals that retain the unpaired electron for a practically
indefinite period of time. These compounds make it possible to extend substantially the possibilities of electron spin resonance as a method of determining metals, raising its sensitivity and broadening the range of the elements determined [ 23. In earlier work [ 1, 23, the characteristic features of such applications were examined in detail, and a review of literature on related problems was presented. In a continuation of the investigation of analytical applications of these compounds [l-6], the analytical properties of the nitroxylcontaining iminomonoxime compound, 4-oximinomethyl-2,2,5,5-tetramethyl-3imidazoline-1-oxyl (I, designated below as temioxime) [ 71 have been studied. 0003-2670/81/0000-0000/$02.50
0 1981 Elsevier Scientific Publishing Company
66
Temioxime is capable of bonding metal ions by coordination to the iminooxy nitrogen and the heterocyclic nitrogen with the formation of a fivemembered ring_ Some of these chelates were synthesized in aqueous medium and investigated by physical methods [ 81. An extraction-radiospectrometric method for the determination of palladium with this reagent was reported [4] . The aim of the work described here was to investigate more comprehensively the analytically important properties of temioxime and to assess its possibilities as a reagent for other metals. This systematic study also provided an opportunity to check the general conclusions about the effect exerted by the spin label on the properties of an extractive reagent that were drawn in the study of spin-labelled p-diketones [l] . EXPERUVIENTAL
Reagent
Temioxime synthesized by one of us (L.B.V.) at the Novosibirsk Institute of Organic Chemistry was purified chromatographieally on a silica gel column (LS 5f40 pm; Lachema, Bmo) using a mixture of ethyl acetate with chloroform (2 + 1) as the mobile phase. The purity of the compound was checked by occasional chromatography on plates with the same grade of silica gel using various organic solvents_ Storage of the reagent for one year did not cause any significant decomposition_ Temioxime solutions were prepared daily by dissolving an accurately weighed portion of the purified reagent in chloroform. The solutions and the solid reagent were stored in dark vessels in a refrigerator.
Extraction
of metals
The extractions of copper, cobalt, nickel, zinc and manganese were investigated by shaking aqueous solutions of the metal with chloroform solutions of the reagent in ground-glass stoppered tubes. Distribution ratios for copper, cobalt, zinc and manganese were determined radiometrically (60Co, 64Cu, 54Mn, 652n)_ The extraction efficiency for nickel was characterized by the decrease of the radioactivity (63Ni) in the aqueous phase in the course of extraction, using a ZhS-7 liquid scintillator.
Chromatography The components of extracts were separated by liquid adsorption chromatography on columns of various sizes containing silica gel LS 5/40 pm. Separation conditions were established by chromatography on a thin layer (0.5 mm) of the same sorbent on 9 X 12-cm glass plates. Chromatograms
were detected with iodine vapour as well as with an ethanolic solution of dimethylglyoxime (detection of nickel-containing spots). Various organic solvents were used as mobile phases. AI1 clzzomatographic experiments were conducted at room temperature.
Instrumentation
E-W_ spectra were recorded with an RE-1306 radiospectrometer at room temperature using ampoules 1 mm in diameter. The g values were determined relative to diphenylpic~~ydr~yl~ magnetic field sweep was checked by manganese(I1) spectra in magnesium oxide powder. The same sample was used as an adjacent concentration standard. Absorption spectra of reagent solutions in the visible and ultraviolet regions were recorded with an SF-16 spectrophotometer with l-cm cuvettes. pH values were measured with a glass electrode using a pH-673 potentiometer. In all experiments the reagents used were of “Pure for Analysis” or “Chemically Pure” grade. Chloroform was purified by six-fold scrubbing with an equal volume of distilled water followed by drying over calcium chloride and distillation.
Procedure
for determination
of cobalt
Place 2 ml of cobalt solution (4 X lo- ‘-1 X IO-* M) in borate medium (pH[ 8.5) into a tube with a ground-glass stopper, add 2 ml of a 10W3M solution of temioxime in chloroform and shake the tube for 5 min. After phase separation, transfer the organic phase to a column with dry LS 5140 I.trnsilica gel compacted as much as possible (bed height 75 mm; diameter 6 mm), Pass a (1 -!- 2) mixture of chloroform and ethyl acetate through the column at a rate of 0.5 ml min-’ under pressure of nitrogen from a gasbag. It is convenient to use long graduated glass columns so that 11 ml of eluent can be added in a single portion and the progress of the elution can be checked easily_ Control the eluent consumption rate by the volume markings on the tube. (Repeated tests showed that this system provided good reproducibility.) Discard the first 7.5 ml of eluate. Collect the subsequent 3 ml; after stirriig, place this solution in an ampoule (1 mm diameter) and record the e.s.r, spectrum at a moduIation amplitude of 1.0 X 1.0 and a h-f. osculations attenua~on of 7 dB, Take the ratio between the amplitude of the complex spectrum derivative and the amplitude of the adjacent standard spectrum derivative (see below) as the measure of signal intensity. For small cobalt contents, evaporate the cobalt-containing fraction of the eluate in a stream of warm air and dissolve the dry residue in 3 ml of chloroform. Determine the cobalt concentration from a previously plotted calibration graph.
Procedure
for determination
of nickel
Place aqueous solutions into a tube with a ground-glass stopper so that
88
the concentrations in the final volume of 2 ml are 0.1 M sodium perchlorate, and 0.01 M sodium acetate, 0.001 M sodium hydroxide and (I-20) X IOm6M nickel. Add 2 ml of 0.01 M solution of temioxime in chloroform and shake
the tube for 40 min. Separate the organic phase, place 2 ml of the same temioxime soiution in the tube and again shake for 40 min. PIace the combined organic phases in a sintered grass filter funnel covered with a 2-mm layer of LS 5/40 pm silica gel protected with a paper filter. Wash the silica gel with 30 mf of ethyl acetate-chloroform mixture (2 + I) and then pass 30 ml of ethanol through it. Evaporate the ethanol solution in a stream of warm air and dissolve the dry residue in 2 ml of chloroform.
Fill an ampoule
(l-mm diameter) with the solution and place it in the resonator of the spectrometer. Record the es-r_ spectrum at a modulation amplitude of 0.7 X 1.0 and a h.f, oscillations attenuation of 7 dB_ Take the ratio between the mean amplitude of the first derivative (for three spectral lines) and the mean amplitude of the first derivative of the adjacent standard lines (see below) as the signal value. Determine the nickel concentration from a previously plotted calibration graph. RESULTS AND DISCUSSION
Recgent behaviour Acid-base properties_ These properties were studied spectrophotometrically_ fn alkaline media, the U.V. spectrum of the reagent has an intensive band with a maximum at 267 nm (Fig. 1); its location is identical with that of the bands of dissociated forms of other oximes f93. From the dependence of the absorbance of the solution at 267 nm on pH, the dissociation constant of the oxime group [lo] (21”C, 1 X 10e4 M temioxime, I = 1 cm, n = 7, CLI = 0.05) was found to be nK% = 9.25 + 0.08 at ionic strength 0.1 (NaClO,) or PC = 9.36 i 0_08. In addition to this acid-base equilibrium for temioxime, protonation of the nitrogren atom in position 3 of the heterocycfe as well as protonation of the nitroxyl group can be assumed. However, it proved impossible to investigate these processes: they seemed to take place at pH values lower than 2, where absorbances were unstable with time. When the spin-labelled @&ketone was investigated [If , it was concluded that the t~~methyl~mid~ol~eoxyl substituent was very electronegative; according to the data obtained, the electronegativity is commensurable with that of the trifluoromethyl group. Comparison of temioxime with other monoximes with respect to the dissociation constant confirms this conclusion. Indeed temioxime has much stronger acidic properties than, e.g., salicylaldoxime (pK, = 12-f), and its acidity is close to that of monoximes containing highly electronegative acyl groups (diacetylmonoxime 9.53; or-furylmonoxime S-67; a-benzy-hnonoxime 8.60). Reagent distribution in the chloroform-water system. Figure 2 shows the dependence of the temioxime distribution ratio (D) on pH in the chloroform-
90
the conclusion on the closeness of molar volume glyoxal aldoxime confiis and phenyl made when flincre,33ents for tetramethylimidazolineoxyl diketones were examined. of metals The extracting capacity of temioxime was tested for five metals: copper, cobalt, nickel, zinc and manganese. The last two were not extracted into chloroform under any conditions, including the presence of strong donoractive compounds and large poorly hydrated ions. Copper. Copper was extracted to a considerable degree into chloroform over a rather broad pH range (Fig. 3)_ The best extraction was achieved at pH 7-10, but even under these conditions and with a 500-fold excess of reagent, not more than 90% of the metal was extracted_ The dependence of copper distribution ratio on temioxime concentration (Fig. 4) shows that it is impossible to improve radically the extraction by a simple increase in reagent concentration. The distribution ratio was not increased by the addition of tributylphosphate. The slopes of the ascending parts of the plots in Figs. 3 and 4, which are close to 2, indicate that the extraction reaction at pH below 9 can be described by
Extraction
Cu2+ + 2HA,,,
= CuAz(,, + 2H’
where H-4 is the temioxime molecule. ~~athematic~ treatment of the experimental data, taking into account competitive complexing with acetate ions in the aqueous phase [11] (pKI = 1.67; pKz = 0.98; pK3 = 0.42; plz, = -0.19 [ 12]), made it possible to obtain the following values of the constants: log K,, = -3.35 c 0.09 (n = 5, 01= 0.05); log KD(CuAz) = 0.91 f 0.03 (n = 14; a = 0.05). Substitution of these
I
4
6
8 OH
10
12
10-4
to-3
10-2 lo-’
Temtoxime concentration (Ml
Fig_ 3, Dependence of copper distribution ratio on pH: IOms M topper( temioxtie; 0.01 M sodium acetate; JX= 0.1 (NaCIO,). Fig. 4. Dependence of copper distribution ratio on temioxime copper; pH 6.8 (0.01 M sodium phosphate); w = 0.1 (NaClO,).
5 X
concentration;
1W3 M lo-’
M
91
values in theequation& = K,K2,(HA)/R~Rn(CuA2) [ll] resulted in a value for the logarithm of the total stability constant for the extracted complex in the aqueous medium: log pz = 15.8 f 0.2. This value can be compared with the stability constants of conventional copper(I1) oximates in Table 1. Copper(I1) temioximate is inferior in stability to dioximates but is close to carbonyloxirnates. Temioxime is also similar to carbonyloximes in the dissociation constant of the oxime group. This confirms the conclusion (made quantitatively in studying spin-labelled P&ketones) that the electron-acceptor effect of the radicalcontaining substituent not only decreases the pK, of the reagent but also causes a corresponding decrease in the stability of the complexes with metals, which, in turn, changes the extractive efficiency. A serious obstacle for the analytical application of this extraction system lies in frequent and unreproducible disturbances of material balance relative to copper that cannot be avoided by introducing weak complexing reagents_ Cobalt. The dependence of the cobalt distribution ratio on pH (Fig. 5) is characterized by a sharp increase in extraction at pH 7.5: over a range of less than 0.2 pH units the distribution ratio increases by more than two orders of magnitude_ The clearly defined plateau observed immediately after the rise is not wide, but the distribution ratios are large: they make it possible to extract in one step more than 97% of cobalt (with a lOO-fold excess of reagent and equal phase volumes). At pH values higher than 9.5, the extraction becomes impossible because of hydrolysis. The dependence of the cobalt distribution ratio on temioxime concentration (Fig. 6) shows that the extraction efficiency cannot be improved by raising the temioxime concentration above a certain proportion; distribution ratios on the plateaux of both plots indicate that the partition constant of the extracted complex is log Ro = 1.75 f 0.05 (n = 10; a! = 0.05). The slope of the ascending part of the plot is 2, showing a 2:l molar ratio between the reagent and the metal in the extracted complex. The cobalt distribution ratios under the optimal conditions are not improved by addition of tributylphosphate (TBP): at small and medium concentrations, TBP had no effect, whereas at 0.1 M concentrations the extraction became worse. The cobalt extraction at pH 8.5 with a loo-fold excess of reagent is not TABLE 1 Stability
constants
of copper
oximates
in aqueous
media
(2S’C)
[ 91
Reagent
COXlditi0Il.S
lotz Pz
Reagent
Conditions
lolz b.
Phenylglyoral aldoxime Diacetylmonoxime a-Benzyhxmnoxime a-Furylmonorine 1-Nitroso-2-naphthol 2-Nitroso-1-naphthol
0.1 M NaClO, .. .. .. .. ..
10.70= 15.91= 13.70= 9.46= 14.40= 15.60=
Nitroso-R-salt Nitroso-HaAt Dimethylglyorime Methylethylglyoxime 1.2Cyclohexanedionedioxime
0.1 M KC1 W-+0 !J +O.l 0.1 M N&IO, 0.2 M N.&IO,
15.oc 13.oc 18.5C 19.6; 20.3s
aDetermined by zn extractionmethod.bDeterminedpotentiometrically.
I
I 4
5
6
7
8
9
IO
PH
Fig. 5. Dependence of cobalt distribution ratio on pH; lo-’ P = 0.1 (NaClO,).
IO
-4
Temoxme
-3 IO
-2 IO
IO
concentmhon
(Ml
-I
M cobalt; lo-” M temioxime;
Fig. 6_ Dependence of cobalt distribution ratio on temioxime concentration; lo+ M cobalt; P = 0.1 (NaCIG,); 5 x lo-’ M sodium tetraborate.
pH 8.4;
affected by very large amounts of borate or even tartrate ions. This permits reliable standardization of extraction pH by using, e.g., a borate buffer. The following conditions can thus be regarded as optimal for cobalt extraction with temioxime into chloroform: pH 8.5 (10m3M borate solution), 100-fold excess of reagent at ionic strength 0.1 (sodium perchlorate). Under these conditions, equilibrium is established in less than 5 min of shaking. Nickel. Radiometric investigation of nickel extraction was hampered by the extremely low radiation energy of the 63Ni radioisotope; liquid scintillation counting was therefore used. The extraction of nickel into chloroform starts only at relatively high pH values (Fig. 7). Since in these conditions (especially above pH 9) there is noticeable hydrolysis, the extraction was conducted in the presence of acetate ions. This made it possible, without any appreciable reduction of recovery, to avoid nickel losses even at pH 11. Extraction efficiency increases with increasing pH, but (if a loo-fold excess of reagent is used) even at pH 10.8, recoveries do not exceed 50%. A 500fold excess of reagent improves the efficiency (Fig. S), but larger excesses give no benefit. Nickel recovery is somewhat increased when more acetate is added. Donor-active compounds (pyridine, TBP) do not affect the extraction efficiency; a slight increase was observed when 1 M levels of additives were used, but phase separation was then poor. The extraction was not rapid: with 2 X 10e6 M nickel and 10d3 M temioxime under the conditions used for Fig. 8, the system reached true thermodynamic equilibrium on shaking for 30 min, the recovery being 75%. The optimal conditions of nickel extraction are: pH 10.8, at least a 500-
93
80. 0 60
40.
20 .I 4
6
8 PH
IO
12
lo* Temioxire
[CT4
1o-3
lo-*
concentmtion
(Mb
Fig. 7. Dependence of nickel extraction on pH; IO-’ M nickel; 0.01 M temioxime; 0.01 M sodium acetate; c1= 0.1 (NaClO,)_ Fig. 8. Dependence of nickel extraction on temioxime concentration; pH 10.8; 2 nickel; 10e4 M sodium acetate; JJ= 0.1 (NaClO,).
X
lo-‘M
fold excess of temioxime, a 500-fold (relative to nickel) excess of acetate ions and 0.1 M sodium perchlorate. About 75% of the nickel is then recovered in one extraction_ After a second extraction, about 95% of the metal is recovered, which is acceptable for a relative procedure to be worked out_ Development mina tions
of OR extraction-radiospectrometric
method
for metal deter-
The necessary prerequisites for the development of methods of metal determinations with the help of a spin-labelled reagent are the completeness and the reproducibility of extraction. Of the three extraction systems investigated, those for cobalt and nickel were suitable for further study. Another important condition is the possibility of separating the signal of the spinlabelled complex of the determined metal from the reagent signal present in the excess [Z] _ The enormous excess of reagent in the extract excludes the possibility of such separations by purely radiospectrometric means. Comparison of the pH dependences of the distribution ratios for the cobalt chelate and temioxime itself shows that there are no extraction conditions suitable for their separation_ Moreover, the excess of reagent could not be removed efficiently by scrubbing the extracts with alkaline solution. The chromatographic separation of the components of the cobalt- and nickel-containing extracts was therefore examined. Cobalt. Chromatography of a cobalt-containing extract in a thin silica-gel layer showed two new components in addition to the excess of temioxime. Thus, when ethyl acetate was used as the mobile phase, the temioxime spot was characterized by Rf = 0.74, and the two new components by Rf = 0.50 and Rf = 0.02. Radiometric investigation proved that both these components contained cobalt, the ratio between their amounts depending on the extraction
94
conditions_ When an aqueous 10m3M cobalt solution (pH 8.5, borate solution) was shaken with 10d3 M temioxime solution in chloroform, the radioactivity ratio between the less and more mobile components was 3:7. However, when cobalt was extracted from lO-6-lO-5 M solutions with a 10q3 M reagent solution in chlcroform, the fraction of component remaining at the’start was approximately constant, amounting to about 3% of the total amount of metal. These cobalt-containing components were isolated from the extract on a sihea gel column by consecutive use of ethyl acetate (separate elution of temioxime and the more mobile coba.lt-containing component) and then acetone (elution of the second component). E.s.r. spectra of these complexes are shown in Fig. 9. The e.s.r. spectrum of the less mobile component is the quintet usual for a nitroxyl~on~~g complex of a diamagnetic ion with exchange interaction of nitroxyls, and coincides with that described for the CoA3 complex (A is temioxime anion) obtained in the aqueous phase (81. The spectrum of the second component (the main one in the conditions of quantitative extraction) has no analogues among the described spectra of nitroxy1containing metal complexes_ This spectrum could have been assigned to the spin-labelled CoAz complex of paramagnetic cobalt(I1) with exchange interaction between the electrons of nitroxyls and the metal ion_ It is, however, difficult then to explain why the compIex should be obtained in greater amounts at lower cobalt concen-
Fig. 9. E.s.r. spectra of extract components after the extraction of cobalt with temioxime: (-----) temioxime; ( -) the main cobalt-containing compound; (----*) admixed(low-
mobility)cobaltcomplex_
95
trations in the aqueous solution; the abrupt rise in the distribution ratio at pH 7.5, and the lack of dependence of the extraction efficiency on donor additives, cannot be explained_ These contradictions can be removed if it is assumed that the extracted compound is an adduct of the chelate with atmospheric oxygen, CoA,O;, where cobalt(fII) is present with oxygen as the 0; anion because of electron density delocalization 1133. This assumption is in agreement with the unusually abrupt change in the distribution ratio with increasing pH, the slope of the log DC0 = f(CnA) dependence, the dependence of the percentage of this compound on .the overall cobalt concentration, and the unusual e.s.r. spectrum. Moreover, this corresponds to the behaviour of coba& in other similar systems, particularly dioxime systems C% 141. It is thus clear that under the conditions of quantitative extraction, the complex which has an e.s.r. spectrum with a quintet is formed only in a relatively small constant proportional amount. This compound is readily separated from the main compound on a chromatographic column. All this makes it possible to develop a method for cobalt determinations based on spectrometry of tkresecond compound only. Attempts TV determine cobalt from the signal of this compound against the background of re agent excess were unsuccessful, although the spectrum contained portions where free tsmioxime absorbed insignificantly. The problem was that complete. separa%on of the signal from the reagent in 100lOOO-fold excess required the use of extremely small amplitudes of magnetic field strength modulation; such amplitudes result in a sharp decline of the sensitivity of the e.s_r. method as a whole. Tests of various mobile phases for the chromatography of the extracts showed that the best separation of the main complex, the reagent excess and the admixed complex was accomplished with a chloroform-ethyl acetate mixture (1 + 2). The Rf values were 0.63 (reagent), 0.21 (main complex) and 0.02 (admixed complex) and the spots were very compact. The extract was separated for e.s.r. spectrometry on a LS 5/40 pm silica gel column (75 mm X 6 mm). The column was dry-filled, with the silica gel being compacted. To speed up the separation, a nitrogen pressure of about 700 hPa was applied at the top of the column. The optimal elution rate was 0.5 ml min-‘. The elution curve obtained (Fig. 10) shows complete removal of the reagent in 7.5 ml of the mobile phase, and elution of the cobalt complex in the next 3 ml. The cobalt-containing fraction of the eluate can be used directly to record the e.s.r. spectrum, but the detection limit can be decreased if the ethyl acetate--chloroform mixture is replaced by pure chloroform after evaporation of the eluate to dryness. The effects of different conditions of e.s.r. spectra recording on the detection limit and the precision of cobalt determinations were investigated. The best results were obtained when the l-mm ampoule was completely filled with solution and submerged in the resonator as far as possible, and when its position was strictly fixed relative to both depth and vertical
96
*i
Fig. 10. Elution curve for the separation of cobalkontaining
extract.
orientation_ The amplitude of the signal from the complex increases monotonously with increasing amplitude of the magnetic field strength highfrequency modulation and decreasing attenuation of the high-frequency oscillations. However, with a fairly open attenuator the noise of the automatic frequency adjustment also increases, and the signal/noise ratio passes through a maximum at an attenuation of 7 dB. The optimal parameters for recording the spectra of the cobalt-containing complex on the RE-1306 radiospectrometer included a 7 dB attenuation of h.f. oscillaticns, and a modulation amplitude setting of 1.0 X 1.0; the time constant and magnetic field sweep rate were chosen depending on the range of cobalt concentrations, but the time of magnetic field sweep from the maximum to the minimum of the first derivative curve was always S-10 times higher than the time constant_ Under these conditions, the complex e.s.r. spectrum of the compound blended into a broad singlet. In the 3 X 1O-7-l X lo-’ M range, the line width did not depend on concentration, and the amplitude of the first derivative was directly proportional to the initial cobalt concentration in the aqueous phase, so that this amplitude served well as a measure of signal intensity. To exclude errors associated with uncontrollable variations in instrumental operation, the signal from the complex was correlated with that of the adjacent standard, manganese(U) in magnesium oxide. The limit of detection for cobalt on the RE-1306 e.s_.r. spectrometer without solvent substitution was 2 x 10e6 M (with respect to concentration in the initial solution prior to extraction). If chloroform is substituted in the final step, the detection limit can be reduced to (3-4) X 10S7 M. The
97
calibration plot for cobalt determination is described by the straight-line equation Z/&tancl = (0.52 + 0.08) X lo6 X Cc, + (0.24 f 0.54)
(n = 5, OL = 0.05)
The accuracy and precision of the method are characterized by the data presented in Table 2; these results were obtained by the faster procedure without solvent substitution. The mean relative error for the determination of 10m5M cobalt from one recording of the spectrum was 6.2%, whereas the error from three parallel recordings was 3.7%. Cobalt determinations were not affected by the presence of tenfold amounts of nickel and insignificantly affected by copper in the same amounts, but iron(II1) and lead interfered seriously. Determinations take 30 min each without solvent substitution and 40 min with solvent substitution. Nickel. Chromatographic investigation of a nickel-containing extract in a thin silica-gel layer showed that the nickel complex is sorbed very securely. The chloroform-ethyl acetate (1 + 2) eluent did not elute the chelate at all, but the excess of reagent moved as a very compact spot (R, = O-63), as shown above. Even such a polar solvent as ethanol eluted the complex only slightly (RI = 0.2). @n this basis, it was decided to use a sintered glass filter funnel to TABLE 2 Results of determining lo-* M cobalt with temiosime by the extraction-xx. No. of parallel tests
Co found from the result of single recording (x 1O-5 M)
1
0.95 0.99 0.86 1.09 1.03 0.96 1.06 0.91 l-10 1.16 0.97 0.99 0.99 1.03 1.08 0.98 0.90 O-94
2 3
4 5
6
method
Co found from the mean amplitude of three recordings (x 1O-5 M)
Relative error of determination from the mean amplitude (W)
1 14 9 3 4 6 9 10 16 3 1
0.94
6
1.01
1
1.05
5
1.04
4
;
1.01
1
0.95
5
Relative error of determination from single recording (a) 5
6 2 10 6
98
which a 2-mm layer of silica gel was applied. After the content of the extract had been sorbed by this filter, the excess of reagent was removed by passing through it 30 ml of the chloroform-ethyl acetate mixture (1 f 2). The complex remaining on silica gel was then washed out with 30 ml of ethanol, the ethanol was evaporated in a stream of warm air, and the dry residue was dissolved in chloroform for e.s.r. spectrometry. The spectrum of the nickelcontaining complex was a triplet of equdUy intense lines whose parameters practically coincided with those of the reagent spectrum (g = 2.0057 F 0.0002; aN = 1150 A m-l; AH,, = 220 m-l). The optimal conditions for the recording of this signal were similar to those for the cobalt determinations, except for a modulation ampiitude setting of O.‘i X 1.0. The calibration plot for nickel determinations by this method is described by the straight-line equation -A /Astind = (0.29 + 0.12) X IO6 X cm + (0.82 -t 1.18) (n = 5, (lr= 0.05) The limit of detection for nickel was 10S6 M. The mean relative error for the determination of 1.5 X 10-’ M nickel was 15% The error of e.s.r. spectrometry itself for concentrations of 2 X 10m6M amounts to 4%, thus the main sources of error are the operations preceding the final measurements. The most probable source of error seems to be incomplete dissolution of the residue in chloroform after the removal of ethanol. Nickel can thus be determined with the help of temioxime, but this method is inferior to the other extraction~.s.r. methods in a number of characteristics (rate of determination, detection limit and precision). CONCLUSIONS This study of the analytically important properties of the new spin-labelled reagent temioxime, has shown that temioxime differs essentially from the previow;y studied spin-labelled reagents in its high stability, which is comparable with that of conventional organic reagents. Suspicion that low stability is a property inherent in all radical-containing reagents is thus disproved. The investigation has confirmed that spin-labelled reagents can effectively extract metals into organic solvents. The evidence obtained confirms the statement that a spin-labelled substituent based on imidazoline has a strong electron-acceptor effect. This results in an increase in the dissociation constant of the chelate-forming group, with a corresponding change in the stability of the complexes with metals, and so influences the completeness and the regularities of metal extraction_ In other respects radical-containing reagents seem to behave as their more usuaf analogues. In the case of temioxime, it again proved impossible to obtain situations where one could, without a significant loss in sensitivity, determine the signal intensity from the complex against the reagent background signal. In using e.s.r. spectrometers of 3.2-cm range, separation of the excess of reagent seems to be a prerequisite for the development of highly sensitive analytical
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methods based on nitroxylcontaining reagents. Investigation of temioxime has demonstrated the possibilities of chromatography as a rapid convenient method of extract separation. Finally, with cobalt and nickel as examples, it has been shown practically that application of the e.s_r. method via the signals of spin-labelled organic radicals provides an advantage in sensitivity even for metals that can be determined by their own pammagnetism [X,16]. REFERENCES
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istry, Applied Science PubIishers,London, 1979, p. 13. 3 P. M. Solozhenkin, N. G. IUassenand F_ A. ShvengIer,Zh. Anal. Khim.. 32 (1977) 2080. 4 S. V. Larionov, V. I. Ovcharenko, R. A. Sadykov and L. B. Volodarskii, IZV. AN SSSR,
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